1. Field of the Invention
The present invention relates to an electromechanical harmonic reflective tuner system, and more particularly to such a system to be used in harmonic load-pull setup for the measurement, characterization and testing of RF or microwave devices. Under high power conditions at its input at the fundamental frequency F0, the device under test (hereinafter referred to as “DUT”) generates an output signal that contains the fundamental frequency F0 and the harmonic frequencies of said fundamental frequency F0. RF/Microwave harmonic reflective tuners are electronic devices or mechanical devices which modify in a predictable way the phase of the reflection of harmonics of a given operation frequency F0. The harmonic reflective tuner has the capability of generating high amplitude gamma to the microwave devices at harmonic frequencies. This technique of subjecting DUT's input and output to variable high gamma phase with corresponding harmonic source tuner and harmonic load tuner, commonly referred to as “harmonic load pull”, is used to test transistors for amplifier, oscillator or frequency multiplier applications specially at high power, when the non-linear effect of the DUT produces harmonic frequencies.
2. Description of Prior Art
The harmonic load-pull setup is composed of an input generator and its associated amplification (4) connected to input tuners, DUT (3), output tuners and the appropriated measurement apparatus (5).
One possible configuration for harmonic load-pull is using frequency discriminators like triplexers at the input of the DUT (7) and at the output of the DUT (7′) shown in FIG. 1 and using large band tuners (6) on all frequency branches, the large band tuners on the harmonic branches being terminated by 50 ohms loads (2) connected to ground (1). With the large band tuners, the impedances at all frequencies at the input and output of the DUT can be controlled independently. The disadvantage of this method lies in the losses of the triplexer, its limited frequency bandwidth and the high number of large band tuners required, 6 in the configuration of FIG. 1.
In order to obviate these problems, a specific harmonic tuner has been proposed in U.S. Pat. No. 6,297,649 issued to Christos TSIRONIS Oct. 2, 2001. Dedicated harmonic tuners are inserted in series between the fundamental tuner (6,6′) and the DUT's (3) at the input, harmonic tuner (8), and at the output, harmonic tuner (8′). These harmonic tuners are comprising a transmission line (9) on which 2 open stubs (11,12) are sliding on the central conductor (10), which open stubs are surrounded by a circular side wall (14,14′) and permanently secured on the said side walls through dielectric, low loss washers (13,13′). In order to eliminate the residual reflection at the fundamental frequency F0, additional open stubs (11′,12′) might be added, said additional open stubs are identical to the first open stubs (11,12). The open stubs are then positioned along the transmission line to control the phase of the reflection as indicated by arrows (11″) and (12″).
The advantages of this harmonic tuner are:                The number of tuners has been reduced to 2.        These tuners are easier to manufacture than fundamental tuner since they do only require 2 horizontal translation control of the resonators along the transmission line longitudinal axis in order to control the phase reflection at harmonic frequencies.        
The disadvantages of this harmonic tuner are:                Stub copper foil sliding on the central conductor with a metallic to metallic contact in order to insure “perfect” galvanic contact to minimize the losses and to increase the band rejection will see these performances significantly degraded with long term use, because of the removal of the gold metallization of the central conductor, therefore decreasing the band rejection and increasing the losses.        
Since the harmonic tuner of U.S. Pat. No. 6,297,649 (8,8′) is inserted in series between the DUT (3) and the fundamental tuner (6,6′), a supplementary constraint on the harmonic reflectors is that they have to be transparent at the fundamental frequency F0.
Since the harmonic tuner of U.S. Pat. No. 6,297,649 (8,8′) is inserted in series between the DUT (3) and the fundamental tuner (6,6′), the losses of the harmonic tuner at the fundamental frequency F0 are directly degrading the performances of the fundamental tuner (6,6′), lowering the gamma tuning range of the fundamental tuners.
Since the harmonic tuner of U.S. Pat. No. 6,297,649 (8,8′) is inserted in series between the DUT (3) and the fundamental tuner (6,6′), the RF isolation at the fundamental frequency F0 is very poor, meaning that any modifications of the position of the harmonic resonators will affect the impedance seen by the DUT at this fundamental frequency F0 and has to be corrected.